This is useful for web developer scenarios because it allows easier sharing of HTML source code snippets. It's also a welcome change from a security point of view, because Internet Explorer 9 is less susceptible to script injection attacks in files delivered with a "text/plain" Content Type.

Use the content type "text/plain" to ensure a plain text rendering in Internet Explorer 9

Configure your server to send proper Content-Type headers for all documents that your server will deliver; for instance, if your server offers Portable Document Format (PDF) files for download, be sure that these files are delivered with the "application/pdf" MIME type

If you find any sites that are sending improper MIME types and behave incorrectly in Windows Internet Explorer, please file a bug on Connect.